A known example of seawater desalination systems is a seawater desalination system that desalinates seawater by passing the seawater through a reverse-osmosis-membrane device (RO-membrane device). In the seawater desalination system, about 1 to 3 mg-Cl2/L of sodium hypochlorite (NaClO) for sterilization is added to seawater. The water is subjected to a dust-removal treatment and subsequently to a pretreatment, in which coagulation is performed by adding an iron inorganic coagulant to the water and filtration is then performed. The pretreated water is pressurized with a high-pressure pump and pressure-fed to the RO-membrane device. The permeate of the RO membrane is drawn as fresh water from which salt has been removed. The concentrate (brine) containing a high concentration of salt is blown down. In order to reduce the degradation of an RO membrane caused by NaClO, sodium bisulfite (NaHSO3) is added to the water on the entry side of the high-pressure pump for removing residual chlorine by reduction and performing an intermittent sterilization treatment. Sulfuric acid is also used for reducing the formation of scale and performing an intermittent sterilization treatment (NPL 1).
Most of the electric power consumed in a seawater desalination plant is used for increasing the pressure with the high-pressure pump. Accordingly, seawater desalination plants include an energy recovery device that recovers energy from the high-pressure concentrate discharged from the RO-membrane device. Examples of the energy recovery device include an energy recovery device that recovers energy from the concentrate by converting the pressure of the concentrate into electric power and an energy recovery device that recovers energy from the concentrate and reuses the energy for pressurization, that is, for pressurizing, with the pressure of the concentrate, the seawater that has been treated with a pretreatment (NPL 1 and NPL 2).
Free-chlorine oxidizing agents, such as NaClO, which have been commonly used for reducing the occurrence of biofouling of RO membranes, cause the oxidation degradation of RO membranes. In order to prevent the oxidation degradation of RO membranes, combined-chlorine slime-control agents, such as chlorosulfamic acid, and stabilized-bromine slime-control agents, such as bromosulfamic acid, have been proposed (PTL 1, PTL 2, and PTL 3).
In the case where a free-chlorine oxidizing agent, such as NaClO, is used, a residual portion of the free-chlorine oxidizing agent is removed by reduction using a reductant such as NaHSO3 on the entry side of an RO-membrane device in order to prevent the oxidation degradation of RO membranes.
Combined-chlorine slime-control agents and stabilized-bromine slime-control agents are used such that a small amount of residual chlorine can be detected on the entry side of an RO-membrane device, since they do not degrade RO membranes. For example, it is described in PTL 1 that it is preferable to use a chlorosulfamate oxidizing agent, which is a combined-chlorine slime-control agent, such that the concentration of the oxidizing agent in the water fed into an RO-membrane device (hereafter, the water to be introduced into and treated with an RO-membrane device is referred to as “RO feedwater”) is 0.1 to 1000 mg/L or, particularly preferably, 1 to 200 mg/L. It is described in PTL 2 that a combined chlorine agent is used such that the total chlorine concentration in the RO feedwater is 1 to 5 mg/L or preferably 1 to 3 mg/L and the free chlorine concentration in the RO feedwater is 0.1 mg/L or less or preferably 0.05 mg/L or less. It is described in PTL 3 that the concentration of available halogen that comes into contact with a separation membrane is preferably 0.01 to 100 mg/L as available chlorine. It is also described that, if the above available halogen concentration is lower than 0.01 mg/L, the formation of slime may fail to be suppressed to a sufficient degree and that, if the above available halogen concentration is higher than 100 mg/L, the degradation of the separation membrane or the corrosion of pipes or the like may occur.
In an RO-membrane treatment system including an energy recovery device that recovers energy from the concentrate of an RO-membrane device, removing residual chlorine by reduction using a reductant, such as NaHSO3, on the entry side of the RO-membrane device subsequent to the addition of a free-chlorine oxidizing agent, such as NaClO, may result in the occurrence of biofouling in the RO-membrane device and the energy recovery device, because residual chlorine, which is effective for sterilization, is not present downstream of the RO-membrane device.
In the case where a combined-chlorine slime-control agent or a stabilized-bromine slime-control agent is used, it is not necessary to remove residual halogen on the entry side of an RO-membrane device, and RO feedwater that contains residual halogen resulting from the combined-chlorine slime-control agent or the stabilized-bromine slime-control agent is introduced into the RO-membrane device. This reduces the occurrence of biofouling in the RO-membrane device, but may result in the occurrence of biofouling in the energy recovery device, into which the concentrate of the RO-membrane device is introduced.
Specifically, although a slime-control agent added to the RO feedwater is concentrated in the RO-membrane device and, in theory, the concentration of the slime-control agent in the concentrate is increased with the water recovery rate of the RO-membrane device, the combined-chlorine slime-control agent or stabilized-bromine slime-control agent becomes decomposed by organic substances deposited on the surface of an RO membrane, organic substances contained in the feedwater, organic substances adhered to a feedwater pipe or a concentrate pipe, and a reductant, such as NaHSO3, that remains in the feedwater, while the water discharged on the raw-water side of the RO-membrane device is introduced into the energy recovery device through the concentrate pipe. As a result, even when the concentration of residual halogen resulting from the combined-chlorine slime-control agent or the stabilized-bromine slime-control agent in the RO feedwater is detected, it may not be possible to detect the residual halogen concentration in the concentrate that is to be introduced into the energy recovery device and to achieve the slime control effect. In such a case, biofouling may occur in the energy recovery device.
If biofouling occurs in the energy recovery device, the concentrate inlet of the energy recovery device becomes clogged. This reduces the energy recovery rate and also incurs the need to frequently stop the operation of the energy recovery device for maintenance in order to unclog the concentrate inlet.
PTL 1: JP 2010-201313 A
PTL 2: WO2011/125764
PTL 3: JP 2015-62889 A    NPL 1: Zousui Gijyutsu Handbook (Handbook of Water Re-use Technology) 2004 (Published on November 25, Heisei 16 (2004), Water Re-use Promotion Center), pp. 408-414    NPL 2: Water Re-use Technology—All About Water Treatment (published on May 10, Showa 58 (1983), Water Re-use Promotion Center)